Crevice corrosion behavior of several super stainless steels in simulated flue gas desulfurization environment
Flue gas desulfurization (FGD) is an important means for thermal power plants and iron and steel industry to effectively control SO2 emission, and is one of the key areas of environmental protection industry. In the past 10 years, FGD industry has developed rapidly, its equipment localization rate has reached more than 90%, the industry market is huge, and the prospect is attractive. FGD equipment system is very complex, the corrosion environment is very harsh, and the metal components often have serious corrosion problems. In this environment, the flue gas temperature changes greatly, and the critical temperature fluctuates, which is easy to condense in the chimney wall and other parts to produce condensed acid [1,2,3]. The gas components of desulfurization flue gas are mainly SO2 and SO3, accompanied by HCl gas, so the condensed acid produced by condensation is usually composed of H2SO4 and HCl. The condensate in desulfurized flue gas is often accompanied by the deposition and dissolution of oxides of iron, copper and other metals, chlorides and other oxidizing substances, so the condensate also has strong oxidizability. The strong acidity of the condensate can accelerate the corrosion of the metal components in the FGD equipment. The oxidizing Fe3 + and Cu2 + chloride can improve the corrosion potential of the metal. In addition, the presence of high concentration of corrosive Cl -, the local corrosion sensitivity of the blunt metal in the condensate will be significantly improved. Therefore, the severe and complex corrosion environment in FGD equipment puts forward higher requirements for its material selection. Stainless steel is a candidate material with high cost performance in flue gas desulfurization equipment, especially the super stainless steel with high alloying and high corrosion resistance has been widely used [4,5,6]. Compared with ordinary stainless steel, super stainless steel has higher strength, toughness, corrosion resistance and stress corrosion resistance. The high content of Cr, Mo and N in the super stainless steel makes the passivation film more stable and has higher resistance to local corrosion. Based on the content proportion of Cr, Mo and N in stainless steel, it is proposed that the pitting resistance index (prEN) is used to semi quantitatively evaluate the local corrosion resistance and the surface passivation film stability of stainless steel . The prEN value is expressed by 1% Cr + 3.3% Mo + 16% n.
According to photographic composition, austenite and duplex super stainless steel are the two main types of super stainless steel. Super austenitic stainless steel (904L, 254SMO, etc.) is composed of a single austenitic phase, and its corrosion resistance is greatly improved compared with ordinary austenitic stainless steel (316). Super duplex stainless steel (2507, etc.) has the same corrosion resistance as super austenitic stainless steel. Because of its unique two-phase structure of austenite and ferrite, its mechanical properties are guaranteed, and its cost is significantly reduced. At present, there are some related researches on the corrosion behavior of super stainless steel in FGD environment. Tian Feng et al.  carried out the actual environmental coupon corrosion experiment in heat exchanger and absorption tower, evaluated the corrosion rate of 2205254smo, 2507 and 316L stainless steel by using the real data in the field, and studied the impact of environmental factors such as SO42 – and Cl – on the corrosion rate in the field. Rajendran et al.  studied 6Mo super austenitic stainless steel, the results showed that the nitrogen segregation had an important effect on the pitting potential rise of stainless steel, and proposed that the super austenitic stainless steel 926alloy and 31alloy could be used as the structural materials of FGD system instead of 316L. In the laboratory simulation study, the dead green solution 11.4% H2SO4 + 1.2% HCI + 1% FeCl3 + 1% CuCl2 (mass fraction) is an ideal solution to simulate the FGD corrosion environment. Among them, FeCl3 + CuCl2 and H2SO4 + HCI with High Cl concentration and strong oxidation can simulate the composition of acid condensate in FGD environment. For example, Zhang Yigang et al.  conducted a systematic study on the corrosion behavior of 304 and 2205 stainless steel in the dead green solution by using the electrochemical experimental method, and successfully evaluated its pitting sensitivity.
For FGD equipment, its components will inevitably have bolt, flange and other gap structures. Due to the different transmission process of the medium in and out of the crevice, there are great differences in the corrosion form between the inner and outer of the crevice, thus inducing crevice corrosion. Crevice corrosion is also a common form of local corrosion of super stainless steel. For example, research  shows that 254SMO super stainless steel flange has serious crevice corrosion behavior in desalination environment, but there is no corrosion damage on the same material of welded pipe fittings.
From the above discussion, it can be seen that the crevice corrosion behavior of super stainless steel in the dead green liquid simulating flue gas desulfurization environment is very likely to occur, and the severity of crevice corrosion may restrict its use in this field. However, the corrosion form, severity and mechanism of crevice corrosion of 904L, 254SMO and 2507, which are the main alternatives for FGD equipment, are still lack of systematic research. Therefore, the crevice corrosion behavior of 904L, 254SMO and 2507 kinds of super stainless steel artificial crevice electrodes in 70 ℃ dead green liquid environment was studied by cyclic voltammetry and scanning electron microscopy (SEM), and the applicability of three kinds of super stainless steel in FGD environment was evaluated. At the same time, the common 316 stainless steel was used as the contrast material.
Using analytical reagent and deionized water to prepare dead green electrolyte solution , the composition of electrolyte solution is (mass fraction): 11.4% H2SO4 + 1.2% HCl + 1% FeCl3 + 1% CuCl2. The experimental temperature is 70 ℃. The electrode materials are 316904l, 254SMO, 2507 and other four kinds of stainless steel. The alloy element composition and pitting resistance index (prEN) of the four kinds of stainless steel are shown in Table 1.
Table 1 Elemental compositions and PREN values of four tested stainless steels (mass fraction / %)
Fig.1 Schematic diagram of the artificial gap electrode: (a) polyethylene bite unit, (b) stainless steel elect-rode plate, (c) physical photograph
Figure 1 shows the structure of artificial gap electrode. Among them, figure 1a is the schematic diagram of the polyethylene occluding part of the artificial gap electrode. Each group of gap electrode consists of two occluding parts, each of which has 20 occluding teeth as artificial gap position. Fig. 1b is the schematic diagram of stainless steel electrode plate with the size of 30mm × 30mm × 5mm. The copper wire is welded on the side of the electrode plate and sealed with high temperature resistant epoxy resin to avoid its participation in the electrochemical process. Before assembling the gap electrode, polish the working surface of the stainless steel electrode plate to 2000 × with sandpaper, and then clean the surface with water and acetone in turn. Use titanium screw rod to fasten the occluding parts on the working surface of stainless steel electrode plate, and the tightening torque is 0.1N · M. Wrap the screw rod with PTFE tape for insulation. Figure 1C is a complete picture of the artificial gap electrode after assembly. Each group of artificial gap electrode has 40 gap positions. In the open cell, Pt was used as auxiliary electrode and SCE as reference electrode. Four kinds of stainless steel artificial gap electrodes were tested by cyclic voltammetry. The scanning rate was 0.166 MV / s. The scanning potential began to scan forward from 100 mV below the open circuit potential until the current density reached 100 mA / cm2. The scanning ended after scanning back to the open circuit potential. The surface morphology of corroded samples was observed by Insecte SEM. The corrosion depth of crevices was measured with a body microscope.
Results and discussion
Figure 2a and b show the cyclic voltammetric curves of 316 and 904L stainless steel artificial gap electrodes in the dead green solution at 70 ℃. It can be seen that in the process of potential forward scanning, the potential first passes through the open circuit potential (about 57 and 214 MV, respectively), and then the polarization current density increases rapidly. During the whole forward scanning process, the electrode surface did not pass through the passivation region. When the current density reaches 100 mA / cm2, the potential begins to sweep back. At the same potential, the sweep current density is significantly higher than the forward sweep current density, forming a lag ring, indicating that local corrosion occurred on the stainless steel surface during the forward sweep of potential [12,13]. In the process of potential sweep back, the sweep back curve does not cross the forward sweep curve, and the area of lag ring is large, which shows that the local corrosion of stainless steel surface is serious, and the passivation film of the material itself is difficult to repair the local corrosion.
Figure 2C and figure d show the cyclic voltammetric curves of stainless steel artificial gap electrode in the dead green solution at 70 ℃. It can be seen that in the process of forward scanning, the potential first passes through the open circuit potential (about 475 and 472 MV), and then in the process of potential rising, the current density is between 0.1 ~ 1 mA / cm2, indicating that the electrode surface is in the state of bluntness at this time. When the scanning potential is higher than 973 and 955 MV, the current density increases rapidly; when the current density reaches 100 mA / cm2, the potential begins to sweep back. The scanning curve formed a hysteresis loop in the process of potential sweep back, and the area of hysteresis loop was significantly smaller than that of 316 and 904L stainless steel. The results show that the local corrosion severity of 254SMO and 2507 stainless steel is significantly less than that of 316 and 904L stainless steel.
Fig.2 Cyclic voltammetric curves of artificial gap electrodes of 316 (a), 904L (b), 254sMo (c) and 2507 (d) stainless steels in death green liquid at 70 ℃
As mentioned above, each group of artificial gap electrode surface has 40 gap positions. According to the different sensitivity of crevice corrosion, the possibility of crevice corrosion exists in every crevice position after cyclic voltammetry test. Therefore, the number and depth of crevice corrosion on stainless steel electrode surface can reflect the ability of crevice corrosion resistance of stainless steel. Figure 3 is a statistical chart of the number of crevice corrosion on the surface of four kinds of stainless steel artificial crevice electrodes after cyclic voltammetry test. It can be seen that there are 40 crevices on the surface of 316 and 904L stainless steel electrodes, 13 crevices on the surface of 254SMO and 19 crevices on the surface of 2507 stainless steel electrodes, respectively.
Fig.3 Statistics of crevice corrosion of four stainless steels
Figure 4 shows the depth statistical diagram of the deepest part of each crevice corrosion pit on the surface of four kinds of stainless steel artificial crevice electrodes after cyclic voltammetry test. The specific algorithm of the vertical coordinate of the statistical chart is: arrange the crevice corrosion pit depth of each stainless steel from large to small and number it I = 1, 2, 3 , N, n are the total number of crevice corrosion pits, and the cumulative probability p = I / (n + 1). It can be seen from the figure that the distribution of the deepest depth of crevice corrosion on the surface of 316 and 904L stainless steel is similar, and the crevice depth of 254SMO is significantly greater than that of 2507 stainless steel (254SMO is slightly deeper than that of 2507 stainless steel). According to the above statistical data of crevice corrosion, the crevice corrosion sensitivity of 316 and 904L stainless steel is the strongest, followed by 2507 stainless steel, and 254SMO stainless steel.
Fig.4 Statistical counts of crevice corrosion depth of four stainless steels
Figure 5 shows the corrosion morphology of 316 stainless steel artificial gap electrode after cyclic voltammetry test in 70 ℃ dead green solution. Figure 5A shows the corrosion morphology of a typical gap on the surface of 316 stainless steel electrode. It can be seen that the edge corrosion of the gap is the most serious. Fig. 5B and E are high magnification pictures of the corrosion morphology of the crack edge. The “lace cover” structure can be observed at the edge of the gap, which is similar to the “lace cover” structure in the metastable pitting process of stainless steel, both of which are composed of the surface passivation film left in the corrosion process. The structure can block the mass transfer between the inside and outside of the corrosion pit in the local corrosion process of stainless steel, thus speeding up the corrosion process. Fig. 5D shows the micro corrosion morphology of the crack edge. It can be seen that the austenite grain boundary is preferentially corroded and the interior of austenite grain is relatively smooth. Fig. 5C shows the micro corrosion morphology near the crack. It can be seen that the corrosion morphology of this part is dense gully, and the corrosion depth is obviously smaller than that of the edge of the gap.
Fig.5 Corrosion morphology of 316 stainless steel artificial gap electrode after cyclic voltammetry in death green liquid at 70 ℃ (a) and the magnified images of the areas I (b), II (c) in Fig.5a, III (d) and IV (e) in Fig.5b
Figure 6 shows the corrosion morphology of 904L stainless steel artificial gap electrode after cyclic voltammetry test in 70 ℃ dead green solution. Figure 6A shows the corrosion morphology of a typical gap on 904L stainless steel electrode surface. It can be seen that the crevice edge corrosion is the most serious. Fig. 6B and Fig. C are the details of the two positions of the crack edge. It can be seen that the gap edge of 904L stainless steel also has a “lace cover” structure, and its size is significantly larger than that of 316 stainless steel. This kind of large-scale “lace cover” structure can hinder the mass transfer process of the inner and outer parts of the gap to a greater extent, and significantly accelerate the corrosion inside the gap. Combined with the statistical results of crevice corrosion depth given in Figure 4, it can be seen that the large-scale “lace cover” structure is the main cause of serious crevice corrosion damage on the surface of 904L stainless steel. Fig. 6e and f show the corrosion morphology of the crack edge corrosion pit bottom, which is similar to 316 stainless steel. The austenite grain boundary is clear, and the interior of the austenite grain is relatively smooth. Fig. 6D shows the micro corrosion morphology close to the crack, which shows uneven and irregular corrosion morphology.
Fig.6 Corrosion morphology of 904L stainless steel artificial gap electrode after cyclic voltammetry in death green liquid at 70 ℃ (a) and the magnified images of the areas I (b), II (c), III (d) in Fig.6a, IV in Fig.6b (e) and V in Fig.6c (f)
Figure 7 shows the corrosion morphology of 254SMO stainless steel artificial gap electrode after cyclic voltammetric test in 70 ℃ dead green solution. Fig. 7a shows the corrosion morphology of a typical gap on the surface of 254SMO stainless steel electrode. Compared with 316 and 904L stainless steel, the crevice corrosion area is smaller. Fig. 7b and C show the corrosion morphology of the crack edge, and it can be seen that the corrosion at the crack edge is the most serious. No “lace cover” structure similar to 316 and 904L stainless steel was observed at the edge of the gap. Fig. 7d shows the micro morphology of crevice edge corrosion. The corrosion morphology of this part is macroscopically smooth and micro pitting. No austenitic grain characteristic corrosion morphology similar to 316 and 904L stainless steel is found.
Fig.7 Corrosion morphology of 254sMo stainless steel artificial gap electrode after cyclic voltammetry in death green liquid at 70 ℃ (a) and the magnified images of the areas I (b), II (c) in Fig.7a, III (d) and IV (e) in Fig.7b
Figure 8 shows the corrosion morphology of 2507 stainless steel artificial gap electrode after cyclic voltammetry test in 70 ℃ dead green solution. Figure 8A shows the corrosion morphology of a typical gap on the electrode surface of 2507 stainless steel. It can be seen that the crevice corrosion damage of 2507 stainless steel is lighter than that of 254SMO stainless steel. Figure 8b and f show the corrosion morphology at the edge of the gap. Similar to other stainless steels, 2507 stainless steel has the most serious corrosion at the edge of the gap. Fig. 8D shows the micro corrosion morphology of the crack edge. It can be seen that the corrosion feature of this part is ulcerative and uneven. Because 2507 stainless steel is a duplex stainless steel, the potential of ferrite phase is lower than that of austenite phase in the acid gap, so they have the tendency of galvanic corrosion. Therefore, the ulcer corrosion morphology of the crack edge should be caused by the prior corrosion of ferrite phase and the residual of austenite phase. Fig. 8C and e show the corrosion morphology of the parts with shallow corrosion depth in the crack. It can be seen that obvious galvanic corrosion morphology can be observed in this part, the austenite phase remains on its surface in strip shape, and the ferrite phase around preferentially corrodes.
Fig.8 Corrosion morphology of 2507 stainless steel artificial gap electrode after cyclic voltammetry in death green liquid at 70 ℃ (a) and the magnified images of the areas I (b), II (c) in Fig.8a, III (d) and IV (e) in Fig.8b
Crevice corrosion is a common form of local corrosion of stainless steel, which is caused by the difference of corrosion environment between the inner part and the outer part of crevice. The reason for crevice corrosion of stainless steel is generally accepted as IR drop mechanism [14,15]. The theory emphasizes that there are differences in oxygen concentration and mass transfer between inside and outside the gap, which lead to local acidification in the gap. The anodic dissolution of metal mainly occurs in the gap, and the metal cations transfer from the gap to the outside of the gap and generate current I. at the same time, the electrolyte in the gap has resistance R. the product IR of the two is the potential difference between the outside of the gap and the inside of the gap. Relevant research shows that in the acidification electrolyte inside the gap, there is an active region between the self corrosion potential and the passivation potential of the polarization curve of stainless steel, in which active dissolution of stainless steel occurs, as shown in Figure 9 [14,15]. When the IR drop inside the stainless steel gap reduces the stainless steel potential inside the gap from the external applied potential (eapp) to the active potential area (Eact), the gap will be anodic dissolved, and the corrosion of the stainless steel gap will be caused, which is usually located at the edge of the gap . According to the corrosion morphology in Fig. 5, 6, 7 and 8, the deepest part of the crevice corrosion pit of the four kinds of stainless steel is the crevice edge part, which shows that the crevice corrosion behavior of the four kinds of stainless steel in the dead green liquid at 70 ℃ can follow the IR drop mechanism.
Fig.9 Schematic diagram of IR reduction mechanism for crevice corrosion of stainless steels[14,15]
In the dead green solution, FeCl3 and CuCl2 make the solution have strong oxidability and stabilize the passivation film on the surface of stainless steel to a certain extent. At the same time, the strong acidity of the solution and the high temperature of 70 ℃ combined with a large number of Cl – can reduce the stability of the passivation film. Without the protection of the passivation film, the strong oxidation of FeCl3 and CuCl2 solution can promote the anodic dissolution of the stainless steel matrix. Therefore, stainless steel gap electrode is faced with complex multi factor corrosion environment in the dead green solution. From the cyclic voltammetric curves, the passivation films of 254SMO and 2507 stainless steel break and induce crevice corrosion after passivation; the area of lag ring of them is small, which shows that the Coulomb equivalent of cations released by crevice corrosion is small and the passivation film is easy to self repair. From the statistical results of crevice corrosion depth and SEM observation results, the crevice corrosion damage of both is also small. The cyclic voltammetric curves of 316L and 904L stainless steel have no passivation zone, but crevice corrosion occurs directly, and the lag ring area of them is large. It can be seen that the Coulomb equivalent of cations released by crevice corrosion is large and the passivation film is difficult to self repair. The self corrosion potentials of the two electrodes are 57 and 214 MV, respectively, compared with those of the saturated calomel electrode. When the current reaches 100 mA / cm2, the potential is 190 and 460 MV, respectively. It can be seen that the crevice corrosion sensitivity of 316 stainless steel is significantly higher than that of 904L stainless steel. According to the statistics of crevice corrosion depth and SEM observation results, the crevice corrosion damage degree of 316 and 904L stainless steel is similar and larger. The main reason is that the scanning current of cyclic voltammetry reaches 100 mA / cm2, and then the scanning is carried out, so that the charge Coulomb equivalent released by crevice corrosion is similar, so the degree of crevice corrosion damage is equivalent. According to the formula of anti pitting index (Table 1), the prEN order of four kinds of stainless steel is 2507 ≈ 254SMO > 904L > 316. The crevice corrosion resistance of four kinds of stainless steel in the green liquid of death at 70 ℃ is almost the same as that of pre. It can be seen that the crevice corrosion resistance of stainless steel in this environment can be significantly improved by increasing the content of Cr, Mo, N and other corrosion-resistant alloy elements.
- (1) 254SMO and 2507 stainless steel have good resistance to crevice corrosion in 70 ℃ dead green solution, while 316 and 904L stainless steel have the most serious crevice corrosion damage. From the perspective of crevice corrosion damage, super stainless steel 904L should be used carefully in FGD equipment.
- (2) In the 70 ℃ dead green solution, the corrosion degree of four kinds of stainless steel is the deepest. It can be seen that the crevice corrosion of four kinds of stainless steel in the simulated FGD environment follows the IR drop mechanism.
- (3) At the edge of the gap, 316 and 904L stainless steels all show the structure of “lace cover”, while 254SMO and 2507 stainless steels, which have strong corrosion resistance to the gap, have no such corrosion morphology.
Source: China Super Stainless Steel Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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